In aromatics alkylation, an aromatic compound, such as benzene, is contacted with an olefin, such as ethylene or propylene, in the presence of a catalyst to produce an alkylated aromatic compound, such as ethylbenzene or cumene. On a commercial scale, it is generally necessary to pass the olefin and aromatic feeds through one or more pretreatment units to reduce the level of certain impurities which are believed to be adsorbed by the process catalysts, reducing their activity. Typically, such pretreatment units include one or more adsorbent beds, conveniently composed of a molecular sieve or activated alumina, which selectively adsorb the harmful impurities in the feed. Each adsorbent bed is cycled between an adsorption cycle in which the bed is used to remove harmful impurities from one or more alkylation feeds, and a reactivation cycle, in which a hot inert gas, such as nitrogen, is passed through the bed to desorb the harmful impurities contained by the bed. Since a single pass of the inert gas through the adsorbent bed is uneconomic, many plants use a gas recirculation system to reduce inert gas usage.
Such gas recirculation systems typically include a compressor, heater, cooler, chiller, and gas/liquid separator Implicit in the design of such systems is the assumption that condensable compounds will be present in the inert gas exiting the pretreatment unit, and that at least a portion of the harmful impurities desorbed during each pass through the pretreatment unit will exit the system in the liquid collected in the gas/liquid separator. The inert gas from the separator is compressed, heated, and returned to the pretreatment unit. Typically only a small portion of the inert gas circulation is purged, so it would be inefficient to reject harmful impurities solely in the purge stream. One problem with this known process is that the harmful impurities may continue to desorb long after all of the condensable compounds have desorbed and condensed As a result, the recycled inert gas may still contain significant quantities of harmful impurities and so may be incapable of effecting complete reactivation of the adsorbent bed. According to the invention, this problem can be ameliorated or overcome, by adding a condensable fluid to the inert gas effluent from the adsorbent beds during reactivation cycles. The condensable fluid provides a medium in which the harmful impurities can be removed in the gas/liquid separator, so that the inert gas recycled to the absorbent bed has a much lower level of impurity than is obtained without the addition of the condensable liquid.